A System for the Dynamic Control and Thermal Characterization of Ultra Low Power Gas Sensors

This paper describes a system for the simultaneous dynamic control and thermal characterization of the heating and cooling phases of an ultralow-power (ULP) micromachined sensor, featuring thermal characteristics that are quite similar to those of innovative ULP semiconducting metal-oxide gas sensors. A pulsewidth-modulated (PWM) excitation system has been realized using a microcontroller featuring an ARM7 core to characterize the thermal behavior of a device formed by a Pt microheater and a Pt temperature sensor, over an insulating membrane. Three operating modes, i.e., constant target heater resistance, constant heating power, and cooling phase monitoring, were implemented. Objectives of the research were to analyze the relation between the time period and duty cycle of the PWM signal and the operating temperature of such ULP micromachined systems, to observe the thermal time constants of the device during the heating and cooling phases, and to measure the total thermal conductance. Experiments indicated that an approximately constant heater temperature in the constant target heater resistance regime (i.e., after the initial thermal transient due to the heating algorithm) can only be obtained if the time period of the heating signal is smaller than 50 mu s, i.e., much faster than the time constant of the device. Constant power experiments show quantitatively a unique time constant tau for both the heater and the temperature sensor (T-sensor) in the heating phase (with a known applied power) and the cooling phase (with zero power). This time constant decreases during heating in a range of 2.3-2 ms as a function of an increasing temperature rise Delta T between the ambient and the operating temperature. Moreover, we observed that, in the chosen operating temperature range, the thermal conductance is a linear function of Delta T. Finally, repeatability of experimental results was assessed by guaranteeing that the standard deviation of the controlled temperature was within +/- 5.5 degrees C in worst-case conditions.